Method and apparatus for extending the bandwidth of a Class D amplifier circuit

ABSTRACT

Class D amplifiers are used for their high efficiency, but they have some undesirable characteristics that limit their useable bandwidth, one of these being the residual switching frequency ripple. Embodiments of the present invention comprise methods and apparatuses for reducing the switching frequency ripple using a technique known herein as ripple steering. The zero ripple class D amplifier is used in a hybrid system comprising a linear amplifier. The hybrid Class D amplifier is modified for extended bandwidth and lowered distortion by the addition of a linear current source feeding output capacitor. The low frequency bandwidth limit on the linear current source is matched to the high frequency bandwidth limit on the Class D current source, resulting in a composite linear/switching current source with bandwidth extended to the high frequency bandwidth limit of the linear current source.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority of U.S. Provisional Application No. 60/565,261 filed on Apr. 26, 2004, entitled “Class D Amplifier with Reduced Output Ripple and Increased Bandwidth”, specification of which is herein incorporated by reference.

FIELD OF THE INVENTION

The present invention relates to Class D amplifiers. More specifically, this invention relates to extending the bandwidth of a class D amplifier circuit.

BACKGROUND OF THE INVENTION

High quality audio power amplifiers are traditionally large, heavy, and inefficient. Typically these equipments are capable of high power audio output with very low total harmonic distortion (THD). However, these equipments achieve only approximately 25% efficiency under normal audio operating conditions because they typically use inefficient linear or quasi-linear amplifiers (e.g. Class A, B, G, and H).

In recent years, the demand for more efficient audio power amplifiers has increased. Thus the shift from Class B to Class D amplifiers for sound reproduction.

Class D amplifiers provide high efficiency, but typically have limited bandwidths, resulting in high THD at high audio frequencies.

A Class D amplifier is basically a switch-mode power supply modified to operate in four quadrants at high frequencies (e.g. audio frequencies). A switch-mode power supply uses pulse-width modulation (PWM) to control the ON/OFF duty cycle of power switching transistor(s) that provide power to a load. The efficiency is high because the switches are not operated in their linear region.

FIG. 1 is an illustration of a simplified Class D topology. As illustrated, a comparator circuit (not shown) inside the Pulse-Width Modulator 110 compares the amplitude of incoming analog audio signal 101 to the amplitude of a reference triangular waveform operating at an intended switching frequency. The comparator circuit switches its output high or low by comparing the incoming audio's amplitude against the amplitude of the triangular waveform. When audio signal 101 is above the amplitude of the triangular waveform, the comparator switches output D+ of the PWM 110 to the ON state. Output D+ remains ON for the duration of time while the input audio signal exceeds the amplitude of the triangular waveform. Conversely, while the input audio signal is below the amplitude of the triangular waveform, the output D+ of the PWM is at the OFF state. Output D− is the inverse, i.e., complementary of output D+.

The relationship between the input audio amplitude and the pulse-width modulator outputs D+, and D− is linear to a first order. The outputs D+ and D− of the comparator drive “totem-poled” transistor switches Q₁ and Q₂. Each transistor switch is a MOSFET device, with a diode device 131 coupled across its terminals to enable four quadrant switching. The topology shown in FIG. 1 is an example embodiment of a class D amplifier where the same type MOSFETs is used for both switches. Alternate embodiments include using complementary transistors, e.g. one p-type and one n-type MOSFET. For this reason the MOSFET symbol used shows no polarity.

Output filter 140 is typically a second order low-pass, e.g., LC configuration filter. The output filter 140 is essential for low pass filtering, or integrating, the carrier's varying pulse width duty cycle for reproduction of the original audio content while attenuating the switching carrier frequency.

For high fidelity audio reproduction, the operating (i.e. switching) frequency of the Class D power amplifier must be significantly higher than the bandwidth of the audio being reproduced. Thus, to reproduce higher bandwidth audio with higher fidelity requires relatively high switching frequency. However, the higher the switching frequency, the more the switching losses (i.e. reduced efficiency). Thus, a prior art Class D system is limited in efficiency and output bandwidth because of the inherent efficiency loss.

As discussed above, in a Class D amplifier, the high frequency switching waveform is filtered with an LC filter, e.g., a second order LC filter. Selection of the filter component values is very important and essential to maximizing performance. For instance, the corner frequency of the LC filter is chosen between the audio bandwidth of interest and the switching frequency. As the corner frequency moves closer to the switching frequency, higher switching frequency ripple content appears at the output. And, as the corner frequency approaches the audio bandwidth of interest, distortion increases in the audio reproduction. Thus, the tradeoff in corner frequency is between higher frequency ripple, and low audio bandwidth, leading to high distortion in the reproduced audio signal.

Although the higher frequency ripple is generally inaudible to the human ear, its presence is still undesirable for several reasons: (1) it is an emissions problem, for instance, it appears as artifacts in the AM radio band and other places; (2) it influences audio measurements; (3) provides a limitation on how clean the output signal looks to the end user; and (4) puts unwanted artifacts on a feedback signal fed to the control circuit, limiting the performance.

Generally, the corner frequency of the output filter is chosen for high bandwidth and low THD thus the compromise is high frequency ripple on the output. This high frequency ripple is a market stigma for Class D amplifiers even though the ripple is outside the audio bandwidth. A detailed discussion of the ripple problem is discussed below.

FIGS. 2 and 3 present a more detailed illustration of the ripple problems associated with prior art half bridge class D amplifiers. FIG. 2 is an illustration of a typical half bridge class D amplifier configuration. The associated waveforms are shown in FIG. 3. As illustrated, the half bridge Class D amplifier configuration comprises essentially of switches Q₁ and Q₂, which are coupled to a modulator; inductor L₁ coupled on one end to switching node 201; output capacitor C₁, which is coupled to the second end of inductor L₁ and across device R_(LOAD); and device R_(LOAD) (i.e. representing the impedance of the loudspeaker) which is coupled to the second end of inductor L₁ and in parallel with capacitor C₁. Two power sources of equal voltage, V_(D), are needed for half bridge operation.

The LC filter (comprising essentially of inductor L₁ and capacitor C₁) is driven by a square wave at the switching frequency. A square wave voltage 310 is generated at the switching node 201 as a result of the modulator 110 driving the gates of transistors (MOSFETs) Q₁ and Q₂. Inductor L₁ integrates square wave voltage 310 into a triangular wave current 320. And finally, the triangular wave current 320 is integrated into a quasi-sine wave voltage 330 by the output capacitor C₁.

The ripple problem is clearly shown in waveform 330. In practice the voltage ripple on a full bandwidth class D amplifier can be on the order of one volt peak-to-peak (1 Vpp) with a fundamental of several hundred kHz, making it extremely prone to interfering with other electronic equipment, especially AM radio receivers. Modulation schemes in which the switching frequency is variable are particularly troublesome.

Prior art Class D systems have used linear amplifiers for error correction in a class D hybrid system. A linear amplifier is advantageous because of its low noise and high bandwidth. However, use of a linear amplifier for error correction in a prior art class D amplifiers becomes very expensive and compromises efficiency because of the size and power of the linear amplifier needed to correct the switching ripple artifact thus reducing the benefit of going to class D in the first place.

The problem with prior art methods is that the switching ripple is typically in the order of 10% of the full output current. Thus, a linear amplifier that would correct for the switching ripple must have 10% of the power of the class D amplifier. For instance, a one kilowatt (1 KW) class D would require a 100 watt linear amplifier to correct for the switching ripple. The 100 watt linear amplifier is expensive and not practical because it gives away the benefit of going to class D in the first instance.

The burden of the 100 watt linear amplifier is large enough that the size, cost, and efficiency of the class D system is severely compromised, and so it is not done in practice. Thus, most error correction schemes, if they are implemented, are left to clean up audio harmonics only up to some limit below the switching frequency. In this manner the linear amplifier can be reasonably sized since switching residual is not impacted.

Moreover, a high quality linear amplifier with the required closed loop performance will require a unity gain cross-over much higher than the switching frequency. For instance, if the required closed loop bandwidth is 20 kHz, then a linear amplifier with unity gain crossover of several times higher (e.g. 1 MHz) would be required to achieve the clean performance at 20 kHz. But with prior art systems, the switching frequency must be below 1 MHz for reasonable efficiency. Therefore, it is not possible with prior art systems to use linear amplifiers to extend the bandwidth of class D amplifiers without requiring the linear amplifier to correct switching ripple. Moreover, a 100 watt linear amplifier eliminates the benefit of the class D and it's just as large as a 1000 watt class D amplifier.

Class D amplifiers are often operated in a full bridge configuration to increase the output power without increasing the power supply voltages. Thus, for completeness, a full bridge conventional class D amplifier is shown in FIG. 4. The two inductors are shown as L_(1A) and L_(1B), and these may be implemented as two discrete inductors or as a single coupled inductor so long as the dot-orientation is consistent. Inductor L_(1B) is coupled to switching node 401 representing the output of MOSFET switches Q₃ and Q₄. The base of MOSFET switch Q₃ is driven by signal D− from the output of the modulator and the base of MOSFET switch Q₄ is driven by signal D+ from the output of the modulator. The output of the class D amplifier is shown with one side grounded, but this is not necessary for operation. The resulting waveforms are similar to that discussed above for the half bridge configuration.

SUMMARY OF THE INVENTION

The invention is a method and apparatus for reducing ripple and extending bandwidth and reducing THD in class D amplifiers. A Class D amplifier is basically a switch mode power supply driven by a high frequency modulator (e.g. a pulse width modulator). Thus, high frequency switching ripple (e.g. high frequency switching noise) is commonplace in prior art class D amplifiers. A second order low-pass filter in the output of the class D amplifier is normally inadequate to provide low harmonic distortion and eliminate ripple. Thus, there is generally a compromise in choosing the corner frequency of the low-pass filter.

Selection of the corner frequency of the output low pass filter involves a tradeoff between high frequency ripple and distortion in the reproduced audio signal. As the corner frequency moves closer to the switching frequency, higher switching frequency ripple content appears at the output. And, as the corner frequency approaches the audio bandwidth of interest, distortion increases in the audio reproduction. Thus, the tradeoff in corner frequency is between higher frequency ripple, which is generally inaudible, and high distortion in the reproduced audio signal. Normally, ripple is compromised in favor of reduced THD.

Thus, class D amplifiers involve an inherent tradeoff between switching frequency and bandwidth. Higher switching frequencies increase bandwidth while decreasing efficiency. A limitation is imposed on the bandwidth of the system due to the class D current source. If the class D current source is operated in a closed loop fashion, then an additional linear current source may be placed in parallel with the class D current source to extend the bandwidth of the closed loop system. The linear current source may have a high pass frequency that corresponds to the low pass (i.e. corner) frequency of the class D current source. The upper limit of the linear current source should be chosen as the desired overall bandwidth of the new hybrid current source.

In one or more embodiments of the current invention, the bandwidth of the current source in a class D amplifier is extended by first closing a loop around the class D current source and then augmenting it with a parallel linear current source. The linear current source may be bandwidth limited with a high pass corresponding to the class D current source roll-off (i.e. corner) frequency. The combined effect of the two current sources effectively results in a single high bandwidth current source, around which a voltage loop is closed.

One or more embodiments of the present invention extend the bandwidth of zero ripple class D amplifiers such as may be used for audio reproduction applications. The zero ripple condition is achieved by adding an auxiliary output to the conventional class D amplifier. The auxiliary output may serve no other useful function other than to steer ripple away from the primary output. Thus, a half-bridge class D would have one auxiliary output while a full bridge circuit may have two auxiliary outputs that may be combined into one. In the full-bridge case, the second auxiliary output may be a floating output.

One or more embodiments of the present invention use a coupled inductor circuit to steer the ripple away from the primary output. The coupled inductor's behavior may be modeled like an ideal transformer with leakage inductance with the primary winding coupled to the main output capacitor and the secondary winding coupled to the auxiliary output. In one or more embodiments, the auxiliary output may be configured as an additional LC circuit.

In one or more embodiments, the main inductor (in the primary output) may be configured as a coupled inductor circuit, a tapped winding, and other magnetically equivalent structures.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration of a simplified Class D topology.

FIG. 2 is an illustration of a typical half bridge class D amplifier configuration.

FIG. 3 is an illustration of the waveforms of a prior art class D amplifier showing output ripple.

FIG. 4 is an illustration of a typical full bridge class D amplifier configuration.

FIG. 5 is an illustration of a coupled inductor circuit as an ideal transformer with lumped parasitic elements being shown with two AC voltage sources connected.

FIG. 6 is an illustration of the coupled inductor circuit redrawn to reflect L_(M) to the secondary side for better visualization of the voltage divider.

FIG. 7 is an illustration of the coupled inductor approach to ripple steering in the half-bridge configuration.

FIG. 8 is an illustration of the full bridge configuration of the coupled inductor embodiment to ripple steering.

FIG. 9 is an illustration of a zero ripple half bridge configuration using a tapped inductor approach.

FIG. 10 is an illustration of the full bridge version of the tapped inductor embodiment to ripple steering.

FIG. 11 is an illustration of a zero ripple half bridge class D amplifier using back-wound coupled inductor configuration.

FIG. 12 is an illustration of the full bridge version of the alternate coupled inductor embodiment to ripple steering.

FIG. 13 is an illustration of sample waveforms of a zero ripple class D amplifier in accordance with an embodiment of the present invention.

FIG. 14 is an illustration of a current mode control loop in accordance with an embodiment of the present invention.

FIG. 15 is an illustration of a linear current source.

FIG. 16 is an illustration of a linear current source operating in conjunction with a zero ripple class D amplifier in accordance with an embodiment of the present invention.

FIG. 17 is an illustration of waveforms showing the bandwidth extension capability of the linear current source operating in conjunction with a zero ripple class D.

DETAILED DESCRIPTION OF THE INVENTION

The invention comprises a method and apparatus for extending the bandwidth of a Class D amplifier. In the following description, numerous specific details are set forth in order to provide a more thorough description of the present invention. It will be apparent, however, to one skilled in the art, that the present invention may be practiced without these specific details. In other instances, well-known features have not been described in detail so as not to obscure the invention.

In general, embodiments of the present invention extend the bandwidth of the class D amplifier by first eliminating switching frequency ripple in the class D amplifier and then using a linear amplifier to compensate primarily for THD artifacts. The THD artifacts is in the order of 1%, instead of the 10% of the prior art. Thus, instead of the 100 watt linear amplifier of the prior art, only linear amplifiers in the range of 10 watts may needed to extend the bandwidth of the class D amplifier using embodiments of the present invention. A ten times reduction in the linear amplifier required for error correction is more manageable and allows for extending the bandwidth of class D hybrids without losing the benefit of the class D system.

Class D amplifiers are used for their high efficiency, but they have some undesirable characteristics, one of these being the tradeoff between switching frequency and bandwidth. High switching frequencies are desirable to increase the bandwidth of the system, which is inversely related to signal distortion. Low switching frequencies are desirable to reduce losses. A limitation is imposed on the bandwidth of the system employing class D amplifiers is due to the bandwidth limitation of the class D current source.

In embodiments of the present invention, the class D current source is operated in a closed loop fashion, and an additional linear current source is placed in parallel with the class D current source for the purpose of extending the bandwidth of the closed loop system. The linear current source acts as a band-pass filter. Thus, by using a linear current source with a low-end corner frequency that corresponds to the high end corner frequency of the class D current source and an upper limit corner frequency at the desired overall bandwidth of the new hybrid current source, the class D system's bandwidth is extended to the upper limit corner frequency of the linear current source. However, bandwidth extension is impractical with the switching frequency ripple of prior art class D systems.

Prior art class D amplifiers require a compromise between ripple and total harmonic distortion (THD) in choosing the corner frequency of the output LC filter. Normally, ripple is compromised in favor of reduced THD. However, the present invention eliminates the need to make the compromise by providing class D amplifiers with the low THD of the prior art but without the ripple. That is, embodiments of the present invention present a high performance class D amplifier with low THD and zero switching frequency ripple (alternately “zero ripple”).

Zero ripple may be achieved by adding an auxiliary output to the conventional class D amplifier. It is not necessary that the auxiliary output serve any other useful function except to remove the ripple from the main output. The half-bridge class D configuration has one auxiliary output while the full bridge circuit may have two auxiliary outputs, which may be combined into one in some embodiments. In the full-bridge zero ripple configuration, the second auxiliary output may be configured as a floating output.

Embodiments of the Class D amplifier of the present invention have greatly reduced output inductor current ripple and greatly reduced output capacitor voltage ripple by steering the ripple through an auxiliary output comprising of an additional LC circuit and a modified main output inductor. The modification to the main output inductor may involve using coupled windings, tapped windings, or other magnetically circuits.

In one or more embodiments of the present invention, a coupled inductor circuit is used to steer ripple away from the main output of the class D amplifier. To visualize how a coupled inductor circuit steers ripple away from a primary output, the coupled inductor is represented as an ideal transformer as shown in FIG. 5.

As illustrated, the principles of a coupled inductor 501 may be represented as an ideal transformer with turns ratio N_(P):N_(S). Where N_(P) is the number of turns on the primary side winding and N_(S) is the number of turns on the secondary side winding. The transformer has a finite magnetizing inductance L_(M); a finite uncoupled inductance L_(P) on the primary side 510; and a finite uncoupled inductance L_(S) on the secondary side 520.

In practice, uncoupled inductance L_(S) may be characterized as the combination of the coupled inductor leakage inductance and a larger discrete inductor. The two sides of the coupled inductor are driven with AC voltage sources, V_(P) and V_(S), where V_(S) may be constrained to be a scalar multiple (“a”) of V_(P), e.g., V_(S)=aV_(P). No other constraints need be imposed on V_(S) and V_(P), thus they may have any waveform and spectrum. These voltage sources create currents, I_(P) and I_(S), that flow into the coupled inductor 501. When the voltage across winding N_(P) is equal to the primary source voltage V_(P), then there is no voltage drop across uncoupled inductance L_(P), and the primary side current I_(P) is therefore equal zero.

When the secondary voltage source V_(S) is scaled down by the voltage divider created by uncoupled inductance L_(S) and the reflected magnetizing inductance L_(M), then scaled up by the turns ratio N_(P):N_(S), it imposes a voltage on the primary winding N_(P) that can be made equal to V_(P), thus satisfying the zero ripple condition. FIG. 6 is an illustration of the coupled inductor circuit redrawn to reflect the magnetizing inductance L_(M) to the secondary side 520 for better visualization of the voltage divider. Zero ripple condition will exist on the primary windings when the primary side voltage V_(P) and secondary side voltage V_(S) are equal so long as there is no voltage drop across the primary windings N_(P), i.e., voltage on N_(P) equals to primary side voltage V_(P). The zero ripple condition is further illustrated below.

As shown in FIG. 6, the magnetizing inductance may be reflected to the secondary side 520 by a transformation equaling the square of the turns ratio. The voltage on the secondary winding N_(S), denoted VNs, may be calculated with the resulting voltage divider as:

${VNs} = {{aVp}\;\frac{{{Lm}\left( \frac{Ns}{Np} \right)}^{2}}{{Ls} + {{Lm}\left( \frac{Ns}{Np} \right)}^{2}}}$

The voltage across the primary winding N_(P), denoted VNp, is calculated by transforming VNs by the turns ratio.

${VNp} = {\left( \frac{Np}{Ns} \right){aVp}\;\frac{{{Lm}\left( \frac{Ns}{Np} \right)}^{2}}{{Ls} + {{Lm}\left( \frac{Ns}{Np} \right)}^{2}}}$

Since the voltage across N_(P) must equal V_(P) for zero ripple, setting VNp to V_(P) and simplifying terms gives:

${Vp} = {\left( \frac{Np}{Ns} \right){{aVp}\left( \;\frac{Lm}{{{Ls}\left( \frac{Np}{Ns} \right)}^{2} + {Lm}} \right)}}$

Further simplifying and rearranging:

${\left( \frac{Np}{Ns} \right){aLm}} = {{\left( \frac{Np}{Ns} \right)^{2}{Ls}} + {Lm}}$

And solving for L_(S):

${Ls} = {{Lm}\frac{\left( {{\frac{Np}{Ns}a} - 1} \right)}{\left( \frac{Np}{Ns} \right)^{2}}}$

Finally, solving for the case when a=1, that is, V_(S)=V_(P):

${Ls} = {{Lm}\left( {\frac{Ns}{Np} - \left( \frac{Ns}{Np} \right)^{2}} \right)}$

Under these conditions, zero ripple occurs at the primary winding. Thus, the coupled inductor approach is one way of solving the ripple problem with class D amplifiers. It would be obvious to those of ordinary skill in the arts that other magnetically equivalent methodologies and circuits may also be employed.

As is illustrated herein, one or more embodiments of the present invention employ the coupled inductor approach to eliminate ripple artifacts in class D amplifiers. FIG. 7 is an illustration of the coupled inductor approach to ripple steering in the half-bridge class D amplifier configuration. As illustrated, the main inductor 710 comprises a primary winding L_(1A) and a coupled secondary winding L_(1C) in parallel. A first end of both windings L_(1A) and L_(1C), i.e., shown as the end without the dot in FIG. 7, is coupled to switching node 201 (i.e., the connection of MOSFET switches Q₁ and Q₂). The second end of winding L_(1A) is coupled to capacitor C₁, which is then coupled to ground. Thus, winding L_(1A) and capacitor C₁ remain configured as the equivalent of the low-pass filter of the prior art (See FIG. 2) with an addition of an auxiliary output using secondary winding L_(1C).

The secondary winding L_(1C) of the coupled inductor 710 forms the basis of the auxiliary output comprising capacitor C₂, and inductor L₂ in which output capacitor C₂ is used primarily for diverting ripple away from the main amplifier (i.e. primary) output, at capacitor C₁. As illustrated, the second end of winding L_(1C) is coupled to one end of inductor L₂, which is coupled to capacitor C₂. Finally, capacitor C₂ may be coupled to ground. Thus, capacitor C₂ steers ripple away from the primary output.

In this illustration, the low frequency voltages on the main output capacitor C₁ and the second output capacitor C₂ are identical, because both outputs track the DC value of the switching node 201. Assuming no substantial AC voltage is present on either output capacitor, the AC voltage at switching node 201 is analogous to both V_(P) and V_(S) as discussed with the illustration of FIGS. 5 and 6. Thus, this embodiment corresponds to the zero ripple condition on the primary windings since the primary side voltage V_(P) and the reflected secondary side voltage across Np are equal.

FIG. 8 is an illustration of the full bridge configuration of the coupled inductor embodiment to ripple steering. As illustrated, the full bridge configuration involves mirroring the half-bridge architecture of FIG. 7 and combining the two sides. However, when two half bridge configurations are coupled together, the resulting circuit will have two L₂ inductors and two C₂ capacitors. In one or more embodiments of the present invention, the two resulting inductors L₂ are consolidated to create one inductor L₃ and the two resulting C₂ capacitors are consolidated into one capacitor C₃ as shown in FIG. 8.

As illustrated in FIG. 8, a first coupled inductor 710 comprising L_(1A) and L_(1C) is coupled at a first end to switching node 201. That is, the first end of winding L_(1A) may be coupled together with the first end of winding L_(1C) and then to switching node 201. A second coupled inductor 820 comprising L_(1B) and L_(1D) is coupled at a first end to switching node 401. Switching node 401 is the coupling point of MOSFET switches Q₃ and Q₄. The dot-orientation of both coupled inductors 710 and 820 are in the same direction and as shown in the illustration.

The second end of winding L_(1A) of coupled inductor 710 is coupled to one end of capacitor C₁. The other end of capacitor C₁ is coupled to ground. In like manner, the second end of winding L_(1B) of coupled inductor 820 is coupled to the other end of capacitor C₁, which is coupled to ground.

For the auxiliary outputs, the second end of winding L_(1C) of coupled inductor 710 is coupled to one end of inductor L₃, which is coupled to one end of auxiliary output capacitor C₃. Finally, the other end of auxiliary output capacitor C₃ is coupled to the second end of winding L_(1D) of coupled inductor 820.

Those of skill in the arts would recognize that the coupled inductor windings L_(1A), L_(1B), L_(1C) and L_(1D) may all exist on the same core as a single integrated magnetic structure, and only a single uncoupled inductor L₃ and secondary output capacitor C₃ are needed. Other embodiments may use two coupled inductors, for instance, L_(1A)–L_(1C) and L_(1B)–L_(1D).

Those of skill in the arts would recognize that there are other configurations that may be magnetically equivalent to those discussed with respect to the embodiments of FIGS. 7 and 8. That is, there may be magnetically other ways to achieve the same behavior discussed with respect to FIG. 7. For instance, since one side of both windings of the coupled inductor is coupled together at the same node, the voltage on the output side of the secondary inductor will be determined by the turns ratio. This voltage ratio may also be realized by the addition of a simple tap on the main inductor winding, as illustrated in FIGS. 9 and 10. This configuration may be an easier winding to manufacture than that discussed with respect to FIG. 7.

FIG. 9 is an illustration of a zero ripple half bridge configuration using a tapped inductor approach. As illustrated, the tapped inductor embodiment involves combining the number of turns that are equivalent between the primary and secondary windings. Thus, as illustrated, the embodiment comprises one inductor L_(1E) with three pins 910, 902, and 903. Pin 903 at the first end of the inductor L_(1E) is coupled to switching node 201. Pin 901 at the other end of inductor L_(1E) is coupled to one end of main output capacitor C₁. The auxiliary output is tapped from pin 902 and coupled to inductor L₂ and auxiliary output capacitor C₂.

The principle of the tapped inductor configuration may be explained using the coupled inductor embodiment of FIG. 7. For example, if the number of turns in the primary windings (e.g. N_(P)) of the coupled inductor 710 is thirty and the number of turns in the secondary windings is twenty four (e.g. N_(S)), then the number of equivalent turns is twenty four. Using the above example, in embodiments that employ the tapped inductor configuration, the main inductor L_(1E) (referring to FIG. 9) will have thirty turns between pins 901 and 903. A secondary tap is included at pin 902, which is located between pins 901 and 903 to represent the number of turns of the secondary winding of L_(1C). In the current example, the tap at pin 902 is located such that there is twenty four turns between pins 902 and 903. Thus, instead of the two windings of the embodiment illustrated in FIG. 7, only one tapped winding may be utilized to magnetically achieve the same result.

FIG. 10 is an illustration of the full bridge version of the tapped inductor embodiment to ripple steering. As illustrated, the full bridge version involves mirroring the half-bridge architecture of FIG. 9 and combining the two sides. However, when two half bridge configurations are coupled together, the resulting circuit will have two L₂ inductors and two C₂ capacitors. In one or more embodiments of the present invention, the two resulting L₂ inductors are consolidated to create one inductor L₃ and the two resulting C₂ capacitors into one capacitor C₃ as shown in FIG. 10. Tapped inductor L_(1F) has a plurality of pins, e.g., 904, 905, and 906. For instance, following the same example discussed above, the number of turns between pins 904 and 906 will be thirty and the number of turns between pins 905 and 906 will be twenty four.

Those of skill in the arts would recognize that the tapped inductor windings L_(1E), and L_(1F) may all exist on the same core as a single integrated magnetic structure, and only a single uncoupled inductor L₃ and a single auxiliary output capacitor C₃ are needed. Other embodiments may use two tapped inductors in separate cores, for instance, L_(1E) and L_(1F).

As further illustrated in FIG. 10, a first tapped inductor L_(1E) is coupled at pin 903 to switching node 201. A second tapped inductor L_(1F) is coupled at pin 906 to switching node 401. The dot-orientation of both tapped inductors L_(1E) and L_(1F) are in the same direction and as shown in the illustration.

Pin 901 of tapped inductor L_(1E) is coupled to one end of capacitor C₁. The other end of capacitor C₁ is coupled to ground. In like manner, pin 904 of tapped inductor L_(1F) is coupled to the other end of capacitor C₁, which is coupled to ground.

For the auxiliary outputs, pin 902 of tapped inductor L_(1E) is coupled to one end of inductor L₃, which is coupled to one end of auxiliary output capacitor C₃. Finally, the other end of auxiliary output capacitor C₃ is coupled to pin 905 of tapped inductor L_(1F).

In another embodiment, a magnetically equivalent circuit involves coupling a winding for the auxiliary output to the output side of the main inductor, and winding backwards in a bucking fashion as shown in FIGS. 11 and 12. FIG. 11 is an illustration of a zero ripple half bridge class D amplifier using back-wound coupled inductor configuration. As illustrated, the main inductor 1110 has a primary winding L_(1A) coupled as a normal class D circuit. That is, a first end of winding L_(1A), i.e., shown as the end without the dot, is coupled to switching node 201 (i.e., the connection of MOSFET switches Q₁ and Q₂). The second end or output end of winding L_(1A) is coupled to capacitor C₁, which is then coupled to ground. Thus, winding L_(1A) and capacitor C₁ remain configured as the low-pass filter of the prior art (See FIG. 2).

The secondary winding L_(1G) forms the basis of the auxiliary output comprising capacitor C₂, and inductor L₂ in which output capacitor C₂ is used primarily for diverting ripple away from the main amplifier output, at capacitor C₁. As illustrated, the second end of secondary winding L_(1G) (the end with the dot) is coupled to the output end of inductor L_(1A) (i.e. node 1101), and back-wound in the same core with winding L_(1A) to the first end. The first end of winding L_(1G) is coupled to one end of inductor L₂, which is coupled to capacitor C₂. Finally, capacitor C₂ may be coupled to ground. Thus, capacitor C₂ steers ripple away from the primary output.

Using the same example as before with the primary side inductor having thirty turns, the secondary side inductor L_(1G) is back-wound six turns thus resulting in effectively twenty four turns. This configuration produces nearly the same effect as if twenty four turns of the secondary inductor were coupled to the switching node (see configuration of FIG. 7).

The embodiment illustrated in FIGS. 11 and 12 also produces the same voltage ratio, but with far fewer turns added, these turns optionally being of a much smaller ampacity with respect to the main winding. In addition, the back-wound method may be better for external leakage field reduction on the coupled inductor if a toroidal core is used. This is because the smaller bucking winding can occupy a smaller portion f the toroid circumference, such that each winding, L_(1A) and L_(1G), both have almost the ideal 360 degree winding coverage, which will reduce leakage flux.

FIG. 12 is an illustration of the full bridge version of the alternate coupled inductor embodiment to ripple steering. As in other embodiments discussed herein, the full bridge version involves mirroring the half-bridge architecture of FIG. 11 and combining the two sides. The secondary windings are coupled from the output nodes 1101 and 1201 instead of the switching nodes 201 and 401. When two half bridges are coupled together, the resulting circuit will have two inductors L₂ and two capacitors C₂. However, one or more embodiments of the present invention consolidates the two resulting L₂ inductors into one inductor L₃ and the two resulting C₂ capacitors into one capacitor C₃ as shown in FIG. 12. Those of skill in the arts would recognize that the coupled inductor windings L_(1A), L_(1B), L_(1G) and L_(1H) may all exist on the same core as a single integrated magnetic structure, and only a single uncoupled inductor L₃ and secondary output capacitor C₃ are needed. Other embodiments may use two coupled inductors, for instance, L_(1A)–L_(1G) and L_(1B)–L_(1H).

FIG. 13 is an illustration of sample waveforms of a zero ripple class D amplifier in accordance with an embodiment of the present invention. The waveforms are from a full bridge zero ripple embodiment (see FIG. 8) running at 130 kHz from a 100–200 VDC supply. The class D circuit is optimized for high power and low audio bandwidth and is similar to the circuit used for generation of the waveforms illustrated in FIG. 3 for the prior art. The main class D output filter is comprised of a 200 uH inductor L₁ and a 5 uF film capacitor C₁. The zero ripple winding comprises N_(P)=30, N_(S)=24, L_(M)=200 uH, resulting in a secondary side uncoupled inductance L_(S)=32 uH. The secondary side uncoupled inductance L_(S) comprises approximately 7 uH leakage inductance and 25 uH discrete inductance, e.g., L₃. Auxiliary output capacitor C₃ may be varied within a wide range (e.g. from 5 uF to 1 uF) with no impact on the operation of the zero ripple class D amplifier.

As illustrated, a square wave voltage 1310 is generated across inductor L_(1A) as a result of the modulator 110 driving the gates of transistors (MOSFETs) Q₁ and Q₂. The current at the output of inductor L_(1A) is shown as waveform 1320. And finally, the voltage output at capacitor C₁ is shown as waveform 1330. In contrast to waveform 330 of FIG. 3 (prior art class D), the ripple characteristic is all but eliminated from waveform 1330. The plots in both FIG. 3 and FIG. 13 are of the same scale.

Note that the values of inductance and capacitance used herein are for illustrative purposes only. Specifically, the circuit used to generate the waveforms of FIG. 13 was optimized for high power and low bandwidth, and thus has a relatively low switching frequency of 130 kHz. And the values of inductance and capacitance reflect this low switching frequency. These values may be much larger than low power full bandwidth circuits applications.

In addition, an additional resonance may exist in the open loop transfer function of the zero ripple Class D amplifier implementation. This additional resonance may be due to the additional inductor and capacitor in the auxiliary output path. This additional resonance may cause a problem when trying to control the system with feedback. Thus, it may be desirable to move the resonance as close to the switching frequency as possible, i.e., out of the way of the closed loop system.

Referring to the half-bridge class D embodiments, the value of the auxiliary output capacitor C₂ (e.g. C₃ for the full-bridge) is at its lower limit when the switching voltage ripple on C₂ interferes with zero ripple operation. Thus simply adjusting C₂ may not be adequate to shift the additional resonance to a desirable frequency (e.g. the switching frequency). However, the resonance can also be moved by simultaneously moving the number of turns in the secondary winding N_(S) very close to the number of turns in the primary winding N_(P) and decreasing the resonant inductor value. In this manner the resonance may be moved to a much higher frequency with little effect on zero ripple Class D amplifier operation.

In one or more embodiments of the current invention, the bandwidth of the current source in a class D amplifier is extended by first closing a loop around the class D current source and then augmenting it with a parallel linear current source. The linear current source may be bandwidth limited with a high pass corresponding to the class D current source roll-off (i.e. corner) frequency. The combined effect of the two current sources effectively results in a single high bandwidth current source, around which a voltage loop is closed.

Extending the bandwidth of the Class D amplifier circuit is possible because the burden of switching ripple correction is reassigned to the power stage using methods discussed herein. Thus, embodiments of the present invention employ error correction schemes of a manageable size because the undesirable artifacts in the zero ripples class D amplifier circuit are in the order of 1% instead of 10% of the output power. For example, a 10 watts linear amplifier for a 1000 watt class D amplifier instead of 100 watts linear amplifier of the prior art.

In Class D, as in switch-mode power supply technology, the problems associated with a second order systems are largely overcome by closing a loop around the inductor current (its state variable) and using this closed current loop as a linear current gain block in a loop closed around the capacitor voltage (its state variable). This is known as current mode control, most commonly implemented as peak current mode control, but the addition of an integrator in the current error amplifier transforms it onto average current mode control. Average current mode control is used when precise control is needed over an inductor current. In either situation the second order resonance is tamed. FIG. 14 is an illustration of a current mode control loop in accordance with an embodiment of the present invention.

As illustrated, the main current mode control loop comprises output inductor 1430 (e.g. L₁) feeding current I_(L1) to output capacitor 1440 (e.g. C₁). Thus, output of inductor 1430, which comprises the closed loop path, i.e., without the feed-forward from Linear Current Source 1420, provides the main current component I_(L1) for charging the output capacitor 1440. Current feedback 1432, which is the state variable of the output inductor 1430, closes the current loop around the class D amplifier thus providing a linear current source for charging the capacitor. As previously discussed, the output inductor 1430 is coupled to the switching node which is driven by the modulator (e.g. 1450). Current Error Amplifier 1460 amplifies the current error in the closed loop system.

In addition, voltage feedback 1442, which is the state variable of the output capacitor 1440, closes the voltage loop around the class D amplifier. The voltage error, i.e. between the input voltage V_(IN) 1401 and the voltage feedback, V_(OUT) 1402 (through block 1442) is buffered by Voltage Error Amplifier 1470 to generate the desired current, I_(PROG). Finally, a linear current source 1420, which may be a smaller linear amplifier, provides the necessary current correction, I_(LINEAR), for errors occurring up to and even past the switching frequency. Thus, the total current charging main output capacitor C₁ is I_(C), which is the sum of the inductor current I_(L1) and the linear current source current, I_(LINEAR). An embodiment of a linear current source is illustrated in FIG. 15.

As illustrated, transistors Q₅ and Q₆ supply current (I_(NODE)) to output node 1510. The circuit comprising transistors Q₇ and Q₈ form a current mirror. Finally, operational amplifier 1520 drives transistors Q₉ and Q₁₀ to modulate the current at I_(NODE) 1510.

The linear current source need not be sized to respond to anything within the class D amplifiers power stage. FIG. 16 is an illustration of a linear current source operating in conjunction with a zero ripple class D amplifier in accordance with an embodiment of the present invention. As illustrated, circuit elements enclosed in dashed block 1610 is a partial representation of a linear current source coupled with the half bridge zero ripple class D embodiment of FIG. 9. The bandwidth extension of the linear current source operating in conjunction with a zero ripple class D is illustrated in FIG. 17.

In this illustration, waveform 1710 represents the amplitude response of the class D amplifier between inductor current I_(L1) and the input current I_(PROG). The frequency response between the linear current source output I_(LINEAR) and the input current I_(PROG) is given by waveform 1720. In operation, the bandwidth of the zero rippled class D current source would extend up to frequency F_(D). The linear current source bandwidth would extend only from the class D current source cutoff frequency F_(D) up to the desired unity gain bandwidth. Thus, since the class D stage operates up to F_(D), the linear amplifier would operate from F_(D) to F_(LINEAR), which is the desired upper corner frequency. Thus, given the hybrid class D amplifier a performance that is equivalent to that of a linear amplifier.

It will be understood that the above described arrangements of apparatus and the method therefrom are merely illustrative of applications of the principles of this invention and many other embodiments and modifications may be made without departing from the spirit and scope of the invention as defined in the claims. 

1. A method for extending the bandwidth of a class D amplifier system comprising: obtaining a class D amplifier circuit with zero switching ripple, said class D amplifier circuit having a first output capacitor; generating a first current source for charging said output capacitor using a current mode control loop around said class D amplifier; generating a second current source for additionally charging said output capacitor, said second current source extending bandwidth of said first current source.
 2. The method of claim 1, wherein said class D amplifier circuit with zero switching ripple comprises: a first set of switches coupled together to form a first switching node; a first coupled inductor circuit having a first primary winding and a first secondary winding, wherein one end of said first primary winding and one end of said first secondary winding are coupled together and to said first switching node; said first output capacitor coupled to said first primary winding and to ground; and an auxiliary output circuit coupled to said first secondary winding.
 3. The method of claim 1, wherein said second current source is a linear amplifier.
 4. The method of claim 2, wherein said first current source for charging said output capacitor comprises output of said first primary winding.
 5. The method of claim 2, wherein said second current source is a linear amplifier.
 6. The method of claim 1, wherein said class D amplifier circuit with zero switching ripple comprises: a first set of switches coupled together to form a first switching node; a first core comprising a first coupled inductor circuit having a first primary winding and a first secondary winding, wherein a first end of said first primary winding is coupled to said first switching node, a second end of said first secondary winding is coupled to a second end of said first primary winding, said first secondary winding is back-wound from said second end to a first end of said first secondary winding in said first core; a first capacitor coupled to said second end of said first primary winding and to ground; and an auxiliary output circuit coupled to said first end of said first secondary winding.
 7. The method of claim 6, wherein said first current source for charging said output capacitor comprises output of said first primary winding.
 8. The method of claim 6, wherein said second current source is a linear amplifier.
 9. The method of claim 1, wherein said class D amplifier circuit with zero switching ripple comprises: a first set of switches coupled together to form a first switching node; a first inductor circuit having a first primary winding from a first end to a second end and a tap at a first intermediate point therein, wherein said first end of said first primary winding is coupled to said first switching node; a first capacitor coupled to said second end of said first primary winding and to ground; and an auxiliary output circuit coupled to said tap at said first intermediate point in said primary winding.
 10. The method of claim 9, wherein said first current source for charging said output capacitor comprises output of said first primary winding.
 11. The method of claim 9, wherein said second current source is a linear amplifier.
 12. A class D amplifier apparatus with extended bandwidth comprising: a class D amplifier circuit with zero switching ripple, said class D amplifier circuit having a first output capacitor; a first current source for charging said output capacitor having a current mode control loop around said class D amplifier; a second current source for additionally charging said output capacitor, said second current source extending bandwidth of said first current source.
 13. The class D amplifier apparatus of claim 12, wherein said class D amplifier circuit with zero switching ripple comprises: a first set of switches coupled together to form a first switching node; a first coupled inductor circuit having a first primary winding and a first secondary winding, wherein one end of said first primary winding and one end of said first secondary winding are coupled together and to said first switching node; said first output capacitor coupled to said first primary winding and to ground; and an auxiliary output circuit coupled to said first secondary winding.
 14. The class D amplifier apparatus of claim 12, wherein said second current source is a linear amplifier.
 15. The class D amplifier apparatus of claim 13, wherein said first current source for charging said output capacitor comprises output of said first primary winding.
 16. The class D amplifier apparatus of claim 13, wherein said second current source is a linear amplifier.
 17. The class D amplifier apparatus of claim 12, wherein said class D amplifier circuit with zero switching ripple comprises: a first set of switches coupled together to form a first switching node; a first core comprising a first coupled inductor circuit having a first primary winding and a first secondary winding, wherein a first end of said first primary winding is coupled to said first switching node, a second end of said first secondary winding is coupled to a second end of said first primary winding, said first secondary winding is back-wound from said second end to a first end of said first secondary winding in said first core; a first capacitor coupled to said second end of said first primary winding and to ground; and an auxiliary output circuit coupled to said first end of said first secondary winding.
 18. The class D amplifier apparatus of claim 17, wherein said first current source for charging said output capacitor comprises output of said first primary winding.
 19. The class D amplifier apparatus of claim 17, wherein said second current source is a linear amplifier.
 20. The class D amplifier apparatus of claim 12, wherein said class D amplifier circuit with zero switching ripple comprises: a first set of switches coupled together to form a first switching node; a first inductor circuit having a first primary winding from a first end to a second end and a tap at a first intermediate point therein, wherein said first end of said first primary winding is coupled to said first switching node; a first capacitor coupled to said second end of said first primary winding and to ground; and an auxiliary output circuit coupled to said tap at said first intermediate point in said primary winding.
 21. The class D amplifier apparatus of claim 20, wherein said first current source for charging said output capacitor comprises output of said first primary winding.
 22. The class D amplifier apparatus of claim 20, wherein said second current source is a linear amplifier. 